The researchers examined the molecular makeup of hamburgers, chicken sandwiches, and french fries purchased from three fast food chains in six U.S. cities.

"Out of the hundreds of meals that we bought, there were only 12 servings of anything that did not go straight back to a corn source," said study lead author Hope Jahren, a geobiologist at the University of Hawaii in Honolulu.

Corn's dominance in the nation's fast food is well known, "but the [chemical analysis] really bring it home in a way that hasn't been brought home before," said Craig Cox, Midwest vice president for the nonprofit Environmental Working Group.

It All Goes Back to Corn

Corn, noted study author Jahren, has a unique biochemistry that allows researchers to identify its signature as it passes through the food chain—from plant to animal tissue to cooked human food, for example.
Jahren and colleagues used this signature to see if the contents of fast food matched how restaurants' said their foods are made and what ingredients they contain.

Such details from Wendy's, McDonalds, and Burger King were "vague and euphemistic," Jahren said.

The team's analysis, however, told a "very simple, straightforward story—it all went back to corn."

French fries are often cooked in corn oil, and cows and chickens eat either corn or feed fertilized with corn.

Not only was the signature of corn dominant, it was remarkably homogenous across the country in different restaurants, she added.